HIV-1 and other lentiviruses encode accessory proteins, each of which plays a role in facilitating virus replication and causing pathogenesis. Several of the accessory proteins exert their effect by counteracting antiviral cell proteins. This project focuses on understanding how the two HIV-related accessory proteins, Vpr and Vpx, facilitate virus replication. These proteins are 50% similar in amino acid sequence, are packaged in the virion, localize to the nucleus and bind a specific E3 ubiquitin ligase. Vpx, which is encoded in HIV-2 and SIVmac but not HIV-1, targets the cellular enzyme SAMHD1 for degradation. The target of Vpr has not yet been identified. The mechanism by which SAMHD1 inhibits viruses is not clear. It is a phosphohydrolase that when expressed in myeloid cells, removes the phosphates from the deoxynucleotide triphosphates, depleting the pool of intracellular dNTPs. SAMHD1 is also an exonuclease and which of these activities restricts virus replication is not clear. SAMHD1 does not block the replication of SIVmac or HIV-2 as these viruses encode Vpx. This project seeks to understand (i) the mechanism by which SAMHD1 restricts HIV-1; (ii) how Vpx binds and targets SAMHD1 for degradation; (iii) how Vpx is released from the virus following infection; (iv) how the enzymatic activity of the protein is regulated by post-translational modifications or by associating with regulatory proteins; (v) which amino acid residues of Vpx and SAMHD1 allow the proteins to interact; (vi) how Vpx is imported into the nucleus and (vii) the role of SAMHD1 in the innate immune response to HIV-1 infection. Lastly, an inducible expression system for Vpr will be established to identify the targeted host factor. The findings will shed light on a new mechanism by which the innate immune system restricts HIV-1 and other human pathogens and how viruses have evolved to escape the restriction. Understanding this may provide strategies to therapeutically enhance the effectiveness of the innate immune response for the treatment of infectious diseases.